In the DCL process, coal is typically slurried with a process derived solvent and pumped up to operating pressure. Typically, the slurry is heated to operating conditions via a heat exchanger or a fired furnace. The heat for heat exchange is typically provided by the heat of reaction during coal liquefaction. In some processes, additional heat is released within the process by the preparation of partially hydrogenated solvent (Donor Solvent) and/or during the upgrading of the liquefaction products to remove sulfur, nitrogen, and oxygen.
Historically, in known DCL processes, the ratio between coal and slurry has been maximized. E.g., U.S. Pat. No. 4,944,866 states “In processes using only process derived products as slurry one objective is to increase the ratio between the coal and slurry as much as possible, in order to keep the expensive reaction space of the liquid phase hydrogenation low”.
Slurry preheat furnaces are used in many patented processes. They are required because the temperature of the mixed slurry is limited because of low proportion of hot process derived solvent relative to the quantity of ambient temperature coal. In addition, processes utilizing a Donor Solvent and/or a low activity catalyst require preheat to temperatures approaching liquefaction conditions to initiate the liquefaction reaction. The combination of these factors results in a requirement for large expensive, preheat furnaces.
An additional problem with the use of slurry preheat furnaces is discussed in “Upgrading of Coal Derived Oil by Integrating Hydrotreatment to the Primary Liquefaction Step” by Graeser, et. al., where it is stated that a fired slurry pretreater system “was very sensitive against deposit and coke formation, especially at the high temperature prevailing in the preheater tubes”.
Similarly, in U.S. Pat. No. 4,666,589, the authors state “It is also known that the coal-oil slurry of finely ground coal and the slurry oil, which is a recycle distillate stream from the operation of a coal liquefaction process, undergoes a swelling stage during heating”. And further, “A great increase in viscosity occurs in the section as a result of the swelling process between the initial heat exchangers for the slurry and the hydrogen gas mixture and the preheater. The increase in viscosity can cause a considerable pressure drop in the absence of special precautions”. In addition, the authors state “When heating the three phase mixture of the coal/oil slurry in the presence of hydrogen-containing gas, sedimentation of the solid components can occur in the heat exchanger pipes of the preheater”. In this patent, the inventors propose using a combination of heat exchangers and furnaces for preheating both the hydrogenation gas and the slurry. In U.S. Pat. No. 4,473,460, a flow scheme is described that utilizes multiple heat exchangers for preheating the slurry and hydrogen during normal operations, without a slurry preheat furnace, for preheating the reactants to coal liquefaction reactor temperature. In this particular scheme, heat is generated from both liquefaction and upgrading.